It has been widely known that the directional stability of a motor vehicle is lost in some cases, depending on the conditions of the road surface, if the wheels of the motor vehicle are locked when the brake is applied sharply. Therefore, it is in practice to control the braking force in such a manner as to eventually keep the slip factor of the wheels at approximately 20%, that is, so as to obtain the maximum coefficient of friction, by controlling the brake fluid pressure in accordance with the changes in the wheel velocity and the wheel acceleration, in consideration of the fact that the wheel velocity declines abruptly immediately before the coefficient of friction .mu. attains the maximum value in relation to the wheels when the pressure of the brake fluid working on the wheel cylinder is increased at the time of braking.
Specifically, motor vehicles are provided with an anti-skid control system which controls the braking force by reducing, increasing, or maintaining the pressure of the brake fluid working on the wheel cylinders in such a manner that the wheels will not be locked at the time of sharp braking.
Such an anti-skid control system is applied either to the control of the front and rear wheels or to the control of the rear wheels, and; in either of these applications, the system needs wheel velocity sensors which detect the rotating speed of at lease the wheels to which the driving force is transmitted, that is, the driving wheels. For the control of the front and rear wheels, such rotating speed sensors are installed for all the individual wheels including the driven wheels, and, for the control of the rear wheels, such a construction would be feasible. Yet, in consideration of various advantages, such as the simplicity of construction, wheel velocity sensors are arranged only on the side of the rear wheels, which are driving wheels.
In this regard, it is noted that a wheel spin occurs at the time of a start or an acceleration on a road surface with a low coefficient of friction, such as a snow-covered road, if excessive driving force is transmitted to the wheels. That is to say, slips will occur not only at the time of braking applied to a motor vehicle, but also at the time of its acceleration, in what is generally called "acceleration slip". In such a case, a system with wheel velocity sensors provided only on the side of the driving wheels will give a detected wheel velocity higher than the actual vehicle speed.
In this regard, the Official Gazette for Patent Laid Open No. 184160-1986, for example, points out the problem, with respect to the existing anti-skid control systems, by a statement to the effect that, in case a wheel spin or the like has occurred while a motor vehicle is running at a generally constant speed on an uneven road, such anti-skid control systems start anti-skid control when the deceleration of the wheels reaches the preset deceleration at the time when the wheel velocity of the driving wheels resumes the original wheel velocity after it increases sharply, so that the brake attains poorer braking efficiency. In an attempt to deal with this problem, the invention laid open in the Official Gazette for Patent cited above proposes the provision of an initial state maintaining means, which maintains the anti-skid control system in its initial phase state while the brake is not working. Specifically, the invention offers a system which resets the anti-skid control circuit while the brake switch remains turned OFF.
However, even with the anti-skid control system disclosed in the Official Gazettes for Patent cited above, it is probable that there emerges a situation in which the wheel velocity is higher than the actual vehicle speed and also the estimated vehicle speed which is set on the basis of the wheel velocity is higher than the actual vehicle speed if a braking operation is performed immediately after the accelerating operation is cancelled when an acceleration slip has occurred. Therefore, the cited system will start its anti-skid control, with a reduction of the brake fluid pressure, at least at the time of a braking operation after an acceleration slip.
FIG. 11 shows one example of the state of control at work in a conventional anti-skid control system. In this Figure, (a) shows the state of the accelerating operation, (b) shows the state of the on/off operations of the brake switch, (c) shows the changes in the wheel velocity Vw, the actual vehicle speed V, and the estimated vehicle speed Vs.sub.0, and (f) shows the changes in the pressure of the brake fluid in the wheel cylinders for the rear wheels. Moreover, the broken line in (d) indicates the fluid pressure output from the master cylinder. The Figure shows a state in which the accelerating operation is cancelled at the point x in the course of an acceleration slip and a braking operation is then performed at the point y, the brake switch being thereupon turned ON. By this series of operations, the wheel velocity Vw decreases sharply as from the point x and an increase of the brake fluid pressure is begun at the point y, but, as the wheel velocity will be lower than the standard speed which is set at the point z on the basis of the estimated vehicle speed Vs.sub.0, the system will start its anti-skid control, performing the operation for a reduction of the brake fluid pressure. That is to say, the estimated vehicle speed Vs.sub.0 is set at a value in excess of the actual vehicle speed V. With this setup, it will take a longer time for the wheel velocity Vw to restore itself to the estimated vehicle speed Vs.sub.0, so that the braking distance is extended.